The one thing I keep missing on my 5k iMac is some form of video-in. Would be great if I could connect my MB Pro to it and use the iMac as a display. Perhaps even in a window and all mouse and keyboard events to the window get passed to the connected MacBook.
This one kills me. With COVID, I'm now stuck working from home. I literally spend all day staring at my work Macbook Pro's small screen with a glorious but unused iMac screen sitting five inches behind it.
Previously, I would sometimes use screen sharing to use the desktop of my MacBook onto my iMac, but due to its slow speed, that is only an emergency measure.